1. Field of the Invention
The present invention relates to a heterojunction field effect type semiconductor device and its manufacturing method.
2. Description of the Related Art
Generally, in a transmitter of a mobile handset, a power amplifier has been required to operate at a lower voltage and have a lower ON-resistance. A GaAs heterojunction field effect type semiconductor device is used as such a power amplifier.
In a first prior art GaAs heterojunction field effect type semiconductor device (see: Yasunori BITO et al., FIG. 2 of “64% Efficiency Enhancement-Mode Power Heterojunction FET for 3.5V Li-Ion Battery Operated Personal Digital Cellular Phones”, 1998 IEEE MTT-S Int. Microwave Symp-Dig., pp. 439–442, June 1998), a channel layer, an undoped AlGaAs Schottky layer, an undoped GaAs layer and a Si-doped n+-type cap layer are sequentially grown by an epitaxial growth process, and a double-recess structure is provided in the Si-doped n+-type cap layer and the undoped GaAs Schottky layer. Then, a gate electrode is formed on the undoped AlGaAs Schottky layer via the double-recess structure, and an ohmic source electrode and an ohmic drain electrode are formed on the Si-doped n+-type cap layer. This will be explained later in detail.
In the above-described first prior art GaAs field effect type semiconductor device, since the double-recess structure is adopted, the ON-resistance can be decreased.
In the above-described first prior art GaAs field effect type semiconductor device, however, since the gate electrode is in direct contact with the undoped AlGaAs Schottky layer, the effective Schottky barrier therebetween is so small, i.e., about 1.0 eV that the gate turn-on voltage Vf is small, i.e., about 0.7V. Therefore, in a normal operation, the gate electrode is forwardly turned ON to create a gate current leakage.
In a second prior art GaAs heterojunction field effect type semiconductor device (see: Shigeki WADA et al., “0.1-μm p+-GaAs Gate HJFET's Fabricated Using Two-Step Dry-Etching and Selective MOMBE Growth Techniques”, IEEE Transactions on Electron Devices, Vol. 45, No. 6, pp. 1183–1189, June 1998), a channel layer, an undoped AlGaAs Schottky layer and a Si-doped n+-type cap layer are sequentially grown by a first epitaxial growth process, and a recess structure is provided in the Si-doped n+-type cap layer. Then, a carbon-doped p+-type GaAs layer is grown on the undoped AlGaAs Schottky layer by a second epitaxial growth process. Then, a gate electrode is formed on the carbon-doped p+-type AlGaAs Schottky layer, and an ohmic source electrode and an ohmic drain electrode are formed on the Si-doped n+-type cap layer. This also will be explained later in detail.
In the above-described second prior art GaAs heterojunction field effect type semiconductor device, since the carbon-doped p+-type GaAs layer forms a p+-n junction with its underlying layers, an effective Schottky barrier against electrons within a channel formed in the undoped InGaAs channel layer is substantially increased. That is, this effective Schottky barrier is increased to a degree of the bandgap of the carbon-doped p+-type GaAs layer such as 1.4 eV.
In the above-described second prior art GaAs field effect type semiconductor device, however, although the effective Schottky barrier is increased from about 1.0 eV to about 1.4 eV by about 0.4 eV as compared with the above-described first prior art GaAs field effect type semiconductor device, the gate turn-on voltage Vf is increased from about 0.7V to about 0.9V by only about 0.2V as compared with the above-described first prior art GaAs field effect type semiconductor device due to the direct contact of the carbon-doped p+-type GaAs layer with the undoped AlGaAs Schottky layer. Therefore, in a normal operation, the gate electrode is still forwardly turned ON to create a gate leakage current.
In a third prior art GaAs heterojunction field effect type semiconductor device (see: K. NISHI et al., “High Current/gm self-Alignment PJ-HFET of Completely Enhancement-Mode Operation”, Extended Abstracts of the 1998 International Conference on Solid-State Devices and Materials, pp. 396–397, 1998), a channel layer, an undoped AlGaAs Schottky layer, an undoped GaAs Schottky layer and a carbon-doped p+-type GaAs layer are sequentially grown at an epitaxial growth process. Also, a gate electrode is formed on the carbon-doped p+-type GaAs layer. Further, an ohmic source electrode and an ohmic drain electrode are formed on the n+-type contact regions. This also will be explained later in detail.
In the above-described third prior art GaAs heterojunction field effect type semiconductor device, no defects are induced in the epitaxially grown carbon-doped p+-type GaAs layer, so that the gate turn-on voltage Vf can be increased to about 1.12V by about 0.22V as compared with the above-described second prior art GaAs heterojunction field effect type semiconductor device.
In the above-described third prior art GaAs field effect type semiconductor device, however, the ON-resistance is large, which will be explained in detail.